DNA replication is essential for cell division, ensuring that each daughter cell receives an exact copy of the DNA.
This process follows the semi-conservative model, where each new DNA molecule has one original strand and one newly synthesized strand.
The steps involved are:
Step-1: Initiation
Origins of Replication
Replication begins at specific sites on the DNA called origins of replication.
Eukaryotic chromosomes have multiple origins to speed up the process.
Unwinding the Double Helix
At these origins, the enzyme DNA helicase unwinds the DNA, separating the two strands and forming a replication fork where DNA synthesis will occur.
Step-2: Primer Synthesis
Requirement for a Primer:
DNA polymerases, which synthesize new DNA strands, cannot start from scratch; they need an existing strand to add to.
Primase Activity
The enzyme primase synthesizes a short RNA primer that provides the necessary starting point for DNA polymerase to begin adding nucleotides.
Step-3: Elongation
Synthesis by DNA Polymerase
DNA polymerase III (in prokaryotes) or DNA polymerase δ (in eukaryotes) adds nucleotides to the 3' end of the RNA primer, elongating the new DNA strand by matching bases (A with T, C with G).
Leading and Lagging Strands
The leading strand is synthesized continuously towards the replication fork, while the lagging strand is synthesized in short segments called Okazaki fragments, away from the replication fork.
Primer Removal and Gap Filling
DNA polymerase I (in prokaryotes) or DNA polymerase ε (in eukaryotes) removes the RNA primers and replaces them with DNA nucleotides.
Step-4: Ligation
Okazaki Fragment Processing
The nicks left between Okazaki fragments on the lagging strand are sealed by the enzyme DNA ligase, resulting in a continuous double-stranded DNA molecule.
Step-5: Termination
Completion of Replication
Replication continues in both directions from each origin until the replication forks meet and merge with forks from adjacent origins or until they reach specific termination sites on the DNA molecule.
Circular Chromosomes
In organisms with circular chromosomes, such as bacteria, replication typically ends at a site opposite the origin called the terminus.
The semi-conservative model ensures that each daughter molecule of DNA has one old strand (from the parental DNA molecule) and one new strand, maintaining the genetic information through successive generations.
This model was elegantly demonstrated by the Meselson-Stahl experiment, which used isotopic labeling to show that DNA replication is indeed semi-conservative.
Inhibitors of DNA Replication
Chemotherapeutic Agents: Drugs like doxorubicin and cisplatin disrupt DNA replication in cancer cells.
Antibiotics: Rifampicin inhibits bacterial RNA polymerase, indirectly affecting DNA replication.
Nucleoside Analogues: Zidovudine (AZT) incorporates into viral DNA, inhibiting replication in viruses like HIV.
Differences Between DNA Replication in Eukaryotes and Prokaryotes
DNA replication is a critical process in both eukaryotic and prokaryotic cells, ensuring that genetic information is accurately passed on during cell division.
While the fundamental principles are similar, there are significant differences in the replication mechanisms between eukaryotes and prokaryotes.
Feature | Prokaryotes | Eukaryotes |
Origin of Replication | Single origin (OriC) | Multiple origins per chromosome |
Replication Rate | ~1000 nucleotides/second | ~50-100 nucleotides/second |
Main DNA Polymerases | DNA Pol III (synthesis), DNA Pol I (primer removal) | DNA Pol α (primer synthesis), Pol δ (lagging strand), Pol ε (leading strand) |
Primer Removal | DNA Pol I | RNase H, FEN1 |
Telomeres | Absent | Present, maintained by telomerase |
Replication Machinery | Simpler, fewer proteins | Complex, multiple proteins |
Chromatin Structure | Nucleoid region, minimal packaging | Nucleus, chromatin with nucleosomes and higher-order structures |
Replication Forks | Single replisome per fork | Multiple replisomes working simultaneously |